Targeted drug delivery means are particularly desirable where toxicity or bioavailability of the pharmaceutical compound is an issue. Specific drug delivery methods and compositions that effectively deposit the compound at the site of action potentially serve to minimize toxic side effects, lower dosing requirements and decrease therapeutic costs. In this regard, the development of such systems for pulmonary drug delivery has long been a goal of the pharmaceutical industry.
The three most common systems presently used to deliver drugs locally to the pulmonary air passages are dry powder inhalers (DPIs), metered dose inhalers (MDIs) and nebulizers. MDIs, the most popular method of inhalation administration, may be used to deliver medicaments in a solubilized form or as a dispersion. Typically MDIs comprise a Freon or other relatively high vapor pressure propellant that forces aerosolized medication into the respiratory tract upon activation of the device. Unlike MDIs, DPIs generally rely entirely on the patients inspiratory efforts to introduce a medicament in a dry powder form to the lungs. Finally, nebulizers form a medicament aerosol to be inhaled by imparting energy to a liquid solution. More recently, direct pulmonary delivery of drugs during liquid ventilation or pulmonary lavage using a fluorochemical medium has also been explored. While each of these methods and associated systems may prove effective in selected situations, inherent drawbacks, including formulation limitations, can limit their use.
Since the introduction of the metered-dose inhaler in the mid 1950s, inhalation has become the most widely used route of administration of bronchodilators and steroids locally to the airways of asthmatic patients. Compared with oral administration of bronchodilators, inhalation via an MDI offers a rapid onset of action and a low incidence of systemic side effects.
The MDI is dependent on the propulsive force of the propellant system used in its manufacture. Traditionally, the propellant system has consisted of a mixture of chlorofluorocarbons (CFCs) which are selected to provide the desired vapor pressure and suspension stability. Currently, CFCs such as Freon 11, Freon 12, and Freon 114 are the most widely used propellants in aerosol formulations for inhalation administration. While such systems may be used to deliver solubilized drug, the selected bioactive agent is typically incorporated in the form of a fine particulate to provide a dispersion. To minimize or prevent the problem of aggregation in such systems, surfactants are often used to coat the surfaces of the bioactive agent and assist in wetting the particles with the aerosol propellant. The use of surfactants in this way to maintain substantially uniform dispersions is said to "stabilize" the suspensions.
Unfortunately, traditional chlorofluorocarbon propellants are now believed to deplete stratospheric ozone and, as a consequence, are being phased out. This, in turn, has led to the development of aerosol formulations for pulmonary drug delivery employing so-called environmentally friendly propellants. Classes of propellants which are believed to have minimal ozone-depletion potential in comparison with CFCs are perfluorinated compounds (PFCs) and hydrofluoroalkanes (HFAs). While selected compounds in these classes may function effectively as biocompatible propellants, many of the surfactants that were effective in stabilizing drug suspensions in CFCs are no longer effective in these new propellant systems. As the solubility of the surfactant in the HFA decreases, diffusion of the surfactant to the interface between the drug particle and HFA becomes exceedingly slow, leading to poor wetting of the medicament particles and a loss of suspension stability. This decreased solubility for surfactants in HFA propellants is likely to result in decreased efficacy with regard to any incorporated bioactive agent.
More particularly, medicament suspensions in propellants tend to aggregate rapidly. If the particle size of the suspended material cannot be regulated and aggregation takes place, the valve orifice of the aerosol container may clog, rendering the dispensing device inoperative or, if a metering valve is employed, it may be rendered inaccurate. This unwanted aggregation or flocculation may lead to improper dosages which can lead to undesirable results, particularly in the case of highly potent, low dose medicaments. Moreover, particle aggregation also leads to fast creaming or sedimentation of the suspension. The resulting phase separation is generally addressed by vigorously shaking the MDI device immediately before use. However, patient compliance is difficult to control and many commercially available suspensions are so unstable that even slight delays between shaking and use can affect dosage uniformity.
Prior art efforts to overcome the difficulties associated with forming stabilized dispersions using environmentally compatible propellants generally involve the addition of HFA-miscible cosolvents (i.e. ethanol) and/or the inclusion of various surfactant systems. For example, several attempts have dealt with improving suspension stability by increasing the solubility of surface-active agents in the HFA propellants. To this end U.S. Pat. No. 5,118,494, WO 91/11173 and WO 92/00107 disclose the use of HFA soluble fluorinated surfactants to improve suspension stability. Mixtures of HFA propellants with other perfluorinated cosolvents have also been disclosed as in WO 91/04011.
Other attempts at stabilization involved the inclusion of nonfluorinated surfactants. In this respect, U.S. Pat. No. 5,492,688 discloses that some hydrophilic surfactants (with a hydrophilic/lipophilic balance greater than or equal to 9.6) have sufficient solubility in HFAs to stabilize medicament suspensions. Increases in the solubility of conventional nonfluorinated MDI surfactants (e.g. oleic acid, lecithin) can also reportedly be achieved with the use of co-solvents such as alcohols, as set forth in U.S. Pat. Nos. 5,683,677 and 5,605,674, as well as in WO 95/17195. Unfortunately, as with the prior art cosolvent systems previously discussed, merely increasing the repulsion between particles has not proved to be a very effective stabilizing mechanism in nonaqueous dispersions, such as MDI preparations.
In addition to the aforementioned surfactant systems, several other attempts have been made to provide stabilized dispersions in environmentally compatible systems. For example, Canadian Patent Application No. 2,036,844 describes the use of suspensions comprising procaterol encapsulated in thermally denatured albumin. Reportedly, the suspensions provide for controlled release of the encapsulated agent. Another attempt at providing stable systems is described in Canadian Patent Application No. 2,136,704 which discloses medicinal aerosol formulations comprising spray dried products and a hydrogenated propellant. The powders apparently contain low levels of a surface active agent to increase particle repulsion and counterbalance attractive forces. Similarly, PCT international Publication No. 97/44012 describes suspension systems comprising powders incorporating low levels of a surface active agent to create "appropriate repulsive forces" that counterbalance electrostatic attractive forces. Yet another system is described in PCT international Publication No. 97/36574 which discusses the use of powders in metered dose inhalers. In these systems it appears that soluble surfactants are added separately to the systems to stabilize the medicament powders. Each of the aforementioned systems is evidently based on the prior art concept that suspension stability is largely achieved by providing repulsive forces that counterbalance the natural particulate attractive forces. Despite such attempts, it is clear that no one has been able to develop a broadly applicable formulation approach that is able to meet the demanding criteria of good dry formulation stability while simultaneously being able to satisfy the ever increasing regulatory standards for MDIs.
Accordingly, it is an object of the present invention to provide methods and preparations that advantageously allow for the efficient delivery of bioactive agents to the pulmonary air passages of a patient in need thereof
It is a further object of the present invention to provide stabilized preparations suitable for aerosolization and subsequent administration to the pulmonary air passages of a patient in need thereof.
It is still a further object of the present invention to provide stabilized dispersions that are compatible for use in a metered dose inhaler and provide reproducible dosing levels over the life of the device.